(1) Field of the Invention
The present invention relates to a digital radio communication system utilizing a multi-value QAM modulation system, and more particularly to a subsignal transmission system.
(2) Description of the Related Art
In a digital radio communication system, one or more protection lines are provided to correspond to a plurality of working lines to prevent deterioration of a transmitted signal due to fading or equipment fault. To switch from one of the working lines to the protection line subsignals such as a control signal superimposed on one of the working lines are used.
Currently, radio communications are effected so frequently that it has been an important goal to improve the utilization rate of frequencies within a predetermined frequency band. One of the techniques to improve the utilization efficiency is to use additional signals in the subsignals such as orderwire signals or way side signals which are also superimposed on one of the working lines.
Conventionally, these subsignals are superimposed on only one of the working lines. On the other working lines, dummy signals having no meaning are superimposed.
However, from the viewpoint of improving the efficiency of utilization of the limited frequency band, it is further required to superimpose subsignals on the other working lines. Although the number of channels for the subsignals is very small in comparison with that of the main signals so that the amount of data which can be transmitted through the subsignal channels is small, the additional signals can be advantageously transmitted between short distances.